ARNOLD L. GORDON, MARTIN VISBECK, and BRUCE HUBER, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964-8000
As part of the international DOVETAIL (Deep Ocean Ventilation Through Antarctic Intermediate Layers) program, 97 conductivity-temperature-depth (CTD) with Lowered Acoustic Doppler Current Profiler (LADCP) and tracer stations were obtained from the Nathaniel B. Palmer (cruise 97-5) from 31 July to 8 September 1997 in the southern Scotia Sea and northern Weddell Sea. Muench ( Antarctic Journal , in this issue) shows the station distributions; the figure shows various parameters along the 45°W section. The data characterize the physical and chemical properties of the dense water outflow from the Weddell Sea, Weddell-Scotia Confluence, and Weddell overflow into the Scotia Sea and provide a snapshot of the velocity field at the time of the CTD stations.
The DOVETAIL CTD/LADCP/tracer data set is very extensive and a number of research topics pertaining to Weddell Sea forced ocean ventilation can be pursued. Preliminary findings are as follows.
The warming of Weddell Deep Water observed during the last few decades within the central Weddell Gyre extends into the northwestern Weddell Sea. Weddell Deep Water warming in that region between 1992 and 1997 amounts to nearly 0.2°C. This trend may result from decrease of deep-water heat loss to the atmosphere and cryosphere or increase of injection of warm circumpolar deep water into the Weddell Gyre.
The DOVETAIL CTD/tracer data along with the 1992 Weddell Ice Station data set nicely define the stratification and spatial pattern of the Weddell Sea Bottom Water benthic layer in the western and northwestern Weddell Sea. The DOVETAIL LADCP provides a glimpse of the velocity field associated with the Weddell Sea Bottom Water. The Weddell Sea Bottom Water benthic layer takes on varied forms: a thick well-mixed layer; a thin, stratified form; and a more complex form having attributes of both types. The transition from thin to thick benthic layer may be aided by diminished importance of the thermobaric effect as mixing of the benthic layer proceeds.
Within the Weddell-Scotia Confluence over the South Scotia Ridge west of the South Orkneys, there is a well-ventilated, low-salinity deep water, which may be referred to as Weddell-Scotia Confluence Deep Water. It advects eastward to provide the bottom water on the southern, deeper parts of the South Orkney Plateau and then passes northward into the Scotia Sea. The DOVETAIL data clearly show it is coming from the Antarctic Peninsula eastern shelf. It may be considered as a less dense form of Weddell Sea Bottom Water. It rides the "outer-rim" of the Weddell Gyre to feed into the Weddell-Scotia Confluence. The Weddell-Scotia Confluence Deep Water spreads on density surfaces into the Powell Basin and Scotia Sea, overriding the Weddell Sea Bottom Water and may influence the thickness of the benthic layer.
The Bransfield basin waters are clearly derived from the freezing-point waters that pass from the Weddell Sea around Joinville Island. The saltiest freezing-point Joinville water contributes to the basin bottom water, whereas the less saline Joinville water contributes to the weak salinity minimum at mid-depth. The warm end member for the deep salinity minimum is derived from the pycnocline water probably coming from the Bellingshausen Sea or southern Drake Passage.
Full ocean depth velocity measurements were made for the first time in the Weddell Sea Gyre and Scotia Sea. The technology used is fairly new and employs two acoustic doppler current profilers (ADCP) mounted on the CTD frame. The instruments measure the velocity shear over a range of 300 meters at a rate of one profile per second. The depth averaged shear profile is then vertically integrated to give a full ocean depth velocity profile with an unknown barotropic mean flow. In combination with accurate ship's positioning (global positioning system) at the beginning and end of each cast, however, the unknown barotropic mean flow can be determined with an accuracy of about 1 centimeter per second. It is also possible to obtain bottom-referenced flow velocities with 300-meter range of the bottom.
The zonal flow along a meridional section at 45°W (figure, block D ) is mainly equivalent barotropic with significant vertical shears only close to lateral boundaries. The flow around Pirie Bank (59°S) is anticyclonic as expected from vorticity dynamics. The largest flow was observed north of the South Orkney Plateau with westward currents exceeding 25 centimeters per second at a depth of 4,000 meters. This boundary current is one of the pathways by which modified Weddell Sea Bottom Water enters into the Scotia Sea. We have estimated a westward transport of about 10 Sverdrup (1 Sv=106 cubic meters per second) north of the South Orkney Islands.
Over the South Orkney Plateau, we expect a strong tidal component in the velocity signal, a component that cannot be resolved by our temporal station spacing. South of the plateau (61.5°S), we encountered the eastward-flowing wind-driven Weddell Gyre boundary current, which has typical flow speeds of 5-10 centimeters per second. The total eastward transport was about 50 Sv. A similar eastward transport was found along the 40°W section.
The circulation in the Powell Basin was cyclonic with a very confined inflow north of the Joinville Ridge. The outflow was broader, and we estimated a total recalculation transport of 18 Sv in the Powell Basin. The rough bathymetry across the gap between the Powell Basin and the Scotia Sea prohibited reliable estimates of overflow transports. Improved knowledge of the bathymetry would have helped in planning and analyzing our velocity data.
The Weddell Sea produces freezing point water covering a wide range of salinity, which on a timescale of only 1 year is advected to the South Scotia Ridge where it ventilates the antarctic circumpolar belt from the seafloor to the Antarctic Intermediate Water density horizon, eventually spreading into the deep and bottom layers of the global ocean. To detect variability of southern ocean overturning and ventilation, a Weddell outflow monitoring strategy needs to be designed and set in place. The DOVETAIL information will be invaluable in the task of establishing a cost-effective, long-term monitoring program.
The research is supported by National Science Foundation grant OPP 95-28807 and (LADCP component) by a grant/cooperative agreement from the National Oceanic and Atmospheric Administration, grant UCSIO P.O. 10075411. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration or any of its subagencies.
Muench, R.D. 1997. Deep Ocean Ventilation Through Antarctic Intermediate Layers: The DOVETAIL program. Antarctic Journal of the U.S. , 32(5).